The provision of electrodes in which an electrode active material is bound by fibrillated polymers and having porosity introduced into the composition has been described in the prior art. U.S. Pat. No. 4,320,185, for example, discloses the production of an electrode of this kind; that patent employs, as the pore forming material, a polymer which is blended with the binder polymer and the electrode active material. This pore forming polymeric material is thereafter removed; this removal of the polymer requires a relatively burdensome and expensive procedure particularly in assuring that the removal of the comparatively difficult to extract polymeric substance is complete. Additionally, the extraction or leaching operation to remove this pore forming polymer has an adverse effect on the density and integrity generally of the resulting electrode composition.
Swelling of the electrode during leaching of the pore forming polymer reduces, interparticle contact within the electrode and thereby reduces the utilization of the electrode in use. Removal of the leaching solvent (e.g., hot water) from the electrode must be undertaken so as to have minimal adverse impact on the electrodes, (e.g., by evaporation); particularly when the electrode will be used as a cathode in a lithium or other alkali metal containing non-aqueous battery. Also small or residual amounts of polymeric pore-formers of the type described in U.S. Pat. No. 4,320,185 can be particularly detrimental in many battery systems as these polymers are soluble and can be converted in the battery by reaction (particularly with the cathode) to compounds which can cause corrosion, enhance self-discharge and foul the cathode and/or the anode.
Various other patents which disclose the formulation of porous sheets utilizing fluoropolymers. For example, U.S. Pat. Nos. 4,089,758; 3,813,461; 4,049,589; 3,962,153; and 4,110,392 deal with porous sheets or films wherein various technologies are used to stretch PTFE thereby producing micropores between the fibrils of PTFE. However, these patents are related to electrode technology only in that these porous sheets can be used as separators (or diaphragms) for electrolytic cells. These patents do not describe the means to support an active material and a conductive material with PTFE and then to form the supported mass into a working battery electrode.
The preparation of porous articles using PTFE is shown in U.S. Pat. Nos. 4,357,262; 3,054,761; 3,890,417; 3,556,161; 4,031,033; 4,238,571; and 4,353,853. These patents generally deal with preparing porous articles with PTFE and PTFE with other components. However, none of these patents describe the preparation of a battery electrode. These patents utilize water emulsions of PTFE to prepare the porous structure. Water emulsions of PTFE contain surfactants to keep the PTFE particles suspended in the liquid. These surfactants introduce the likelihood of a detrimental effect on the performance of the electrode. It is also difficult to control the distribution of materials using the slurry formulation methods employed with these PTFE emulsion. In no case do any of the methods employed in these patents use or recognize the necessity of a high shear mixing operation to fibrillate the PTFE thereby causing the mixture to become cohesive. Development of this cohesive property is necessary to formulation of a suitable electrode. Some of the patents in this group use heat or calendaring to achieve some fibrillation, but this does not achieve the required high a degree of fibrillation needed and which can be achieved in the process described herein. Thus, while considerable literature is available describing the formulation of electrically active aggregate which is bound by a fluoropolymer and wherein porosity is introduced into the bound composition, no prior art is known which affords a means for providing an improved electrode wherein the porosity is introduced with little or no disruption in the made up or shaped electrode sheet or article.